Portable Barrier and Rock Fall

Question

State

MN

Description Text

We have a temporary work zone situation, where the designer would like to use portable concrete barrier for a potential rock fall concern. Most of the rock fall (95%) will be contained within the ditch section, but 5% could make it to the traveled lane.

The designer understands the concern over pinning barrier on the opposite side of traffic. But feel that it will be placed far enough away from the 30 mph work zone traffic to be a concern.

This is an interesting question and would appreciated you guidance on how the barrier would react to the forces describe below.

Currently our rock fall analysis is finding that a maximum of 5000 ft/lbs of total kinetic energy (translational plus rotational) could impact the barrier at 90°, typically at the base of the barrier or as high as 1' above the base of the barrier. Would J-barrier, which is pinned on the side being impacted, be able to withhold that impact? If it was not pinned what would the deflection be?

Keywords

Temporary Barriers

Other Keywords

none

Date

January 5, 2016

Response

Response

I have briefly reviewed the inquiry regarding the rockfall concerns near roadways and the use of PCBs for debris containment. Below, it was noted that the impact energy for the design scenario would be approximately 5 kip-ft (6.8 kJ) when applied perpendicular to a portable concrete barrier (PCB) system. Initially, this kinetic energy seems to be rather low. However, I would like to give some perspective to a known quantity that is similar to the roadside safety community.

Let us for the moment consider the AASHTO MASH impact conditions for evaluating roadside barrier hardware. A Test Level 1 (TL-1) impact condition involving a small car would provide an impact severity of 14.0 kip-ft (18.9 kJ), which consists of a 2,425-lb sedan striking at 31.1 mph and 25 degrees. Normally, a TL-1 condition would also include a 5,000-lb pickup truck impact event as well, which is of course the strength test. Most PCB systems are tested and evaluated at a TL-3 condition, which includes a 62 mph (100 kph) impact speed at a 25-degree angle. PCBs are designed to meet the TL-3 impact condition, which provides an impact severity of 115 kip-ft (156 kJ) for the pickup truck with up to 80 in. of dynamic displacement.

It should be noted that the design condition of 5 kip-ft (6.8 kJ) is significantly lower than that provided by the lower of two impact scenarios for TL-1. The design condition would be somewhat represented by a 2,425-lb small car impact the PCBs at a speed of 18.6 mph (30 kph) and 25 degrees.

Based on this fundamental comparison and knowledge of PCB performance under impact events, I believe that existing PCB systems could contain the rockfall material under design conditions when un-pinned to a foundation. Further, dynamic deflections would be estimated as 3.5 in. for this design condition.

In addition, I also reviewed a research report that was conducted by the University of Akron for the Ohio DOT, titled Rockfall Concrete Barrier Evaluation and Design Criteria. Within this study, the OHIO DOT PCB system was evaluated. Researchers determined that the Ohio PCB system could contain rockfall material with a kinetic energy of 18.2 kip-ft (24.7 kJ) with observed sliding of approximately 12 to 15 in. Since the Ohio PCB is similar to that used by the MnDOT, this research confirms that a rockfall slide with material having 5 kip-ft of energy would be safely contained. For a reduced impact energy, deflections may be estimated as 3.3 to 4.1 in.

Using both methods, I can confidently state that the deflections should be less than 6 to 8 in. when un-pinned and connections pulled taught. Also, I do not see a need to pin the barriers to a foundation due to this low estimated deflection.

Please let me know if you have any questions or comments regarding the information contained above. Thanks!

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